An Accelerating Universe: The 2011 Nobel Prize in Physics

Listen to the announcement of the 2011 Nobel Prize in Physics, to Saul Perlmutter, Brian Schmidt and Adam Riess, from the Royal Swedish Academy of Sciences. Following the formal announcement comes an explanation of the research, which tracked type Ia supernovae to discover that the expansion of the universe was accelerating, and a phone conversation with new Nobel laureate Brian Schmidt

Steve: Welcome to a special edition of the Scientific American podcast, Science Talk. Often Nobel Prize announcements can be fairly difficult to understand, but this morning's announcement of the 2011 Noble prize in physics was actually fairly intelligible even to us average lay people. Here is the entire announcement from the Royal Swedish Academy of Sciences of the 2011 Nobel Prize in physics. The first voice you hear will be Staffan Normark.

Normark: This year's Nobel Prize in physics is about our entire universe. The Royal Swedish Academy of Sciences has decided to award the 2011 Nobel Prize in Physics with one half to Professor Saul Perlmutter, the Supernova Cosmology Project, Lawrence Berkeley National Laboratory and University of California, Berkeley, U.S.A., and the other half jointly to Professor Brian Schmidt the High-Z Supernova Search Team, Australian National University Western Creek Australia and Professor Adam Riess, the High-Z Supernova Search Team, John Hopkins University and Space Telescope Science Institute, Baltimore, U.S.A. And the Academy citation runs: "For the discovery of the accelerating expansion of the universe through observations of distant supernovae." Professor Börje Johansson will now give us a short summary in English, please.

Johansson: This discovery of the accelerating expansion of the universe is a milestone for cosmology. The expansion history of the universe gives us insights into the evolution of the universe and possibly about the ultimate fate of the universe. The expansion of the universe was discovered already in the 1920s. The expansion rate depends on the energy content. A universe containing only matter, should eventually slow down due to the attractive force of gravity. Everybody expected that the expansion rate of the universe should slow down. Two independent research groups, one led by Saul Perlmutter and the other one by Brian Schmidt with Adam Riess as a crucial member, made observation of supernovae at distances over about 6 billion light years. From these studies they found that presently the expansion rate of the universe is accelerating, increasing. This conclusion came as a complete surprise for the scientific community. The Supernova Cosmology Project was initiated in 1988 by Saul Perlmutter with the aim of measuring the presumed deceleration, slowing down, of the universe. Brian Schmidt organized in 1994 the competing collaboration: the High-Z Supernova Search Team; a key role in the team was played by Adam Riess, then a post doctor. Now Professor Olga Botner will give more details about the background to the prize. Please…

Botner: Our universe is expanding and accelerating. The fact that the universe is expanding we have known for the best part of a century. And what you see in the illustration here is the expanding universe with the timeline running at the bottom, starting at the big bang about 14 billion years ago and continuing until the present time, now. Galaxies are drifting apart embedded in the fabric of the spacetime just like raisins in a raisin cake which is swelling in the oven. The expansion is described by Einstein's general theory of relativity and is driven by gravity. To understand how gravity influences the expansion, we need to study the expansion rate of the universe over the past billions of years. In a universe which is dominated by matter, one would expect that the pull of gravity eventually should make the expansion slow down. Imagine then, the amazement, the utter astonishment when two groups of scientists headed by this year's Nobel laureates in 1998 discovered that the expansion was not slowing down, it was actually accelerating. The unreal feeling could be likened to the feeling you would get if you in your car, step on the brake and suddenly realize that your car is actually accelerating. But back to the acceleration—how was it discovered? The two groups studied a certain kind of supernovae called supernovae type Ia. These supernovae signaled the death of a certain kind of stars called white dwarves. A white dwarf is what you see illustrated here; it's a star which starts its life with a mass similar to the mass of our sun, but which when it has stopped burning fuel, shrinks to the size of the Earth. We believe that sometimes when it has a companion, it can pull matter off the companion increasing its own mass. When a critical limit is exceeded—and here you see the limit, which is about 1.4 solar masses—runaway fusion processes start in the interior of the star, and the entire star is ripped apart in seconds. An immense amount of energy is released, and we observe this energy release as a supernova. For a brief period of time, a period of just a few weeks, such a supernova, like the one you see here, can outshine an entire galaxy. Type Ia supernovae, as they are called, have all about the same mass when they explode, therefore they release about the same amount of energy. We say that they have the same intrinsic brightness or luminosity. However, the brightness, which we observe at Earth, depends on their distance, the further away they are, the fainter they are. And so by observing brightness at Earth and comparing to the expected brightness, we can determine the distance. Heavenly objects, which can be used in that way to determine distance from observed brightness, are called standard candles. The universe is vast however, and supernovae type Ia are rare; and it was only in the 1990s that the two teams of scientists were able to develop efficient methods, based on CCD imaging, to observe thousands of galaxies in a single scan, and in that way being able to find tens of supernovae per scan. Time could be scheduled in advance at the world's largest telescopes, and the supernovae could be identified before they faded away. Normally we expect that in a thousand years, only one or two supernovae can be identified per galaxy, and the last one observed in the Milky Way was in 1572—the Tycho supernova. So what was the discovery? Well, by comparing the brightness of distant, faraway supernovae with the brightness of nearby supernovae, the scientists discovered that the faraway supernovae were about 25 percent too faint—they were too far away. The universe has not been slowing down, as was expected; it was accelerating. So what makes the universe speed up? Well, actually we don't know. This is a hot research topic and the hottest candidate is what is named dark energy. We don't know what it is, but we know that to account for the observed acceleration rate of the expansion, the dark energy must amount to about 75 percent of the total energy density of the universe. And so the observation of the accelerating expansion of the universe has changed our understanding of the universe. We now realize that our universe, to 95 percent, consists of objects of which we don't know anything—the dark matter and the dark energy. Only 5 percent is stars and planets, flowers and bees and ourselves. And so this discovery is fundamental and a milestone for cosmology, and a challenge for generations of scientists to come. Thank you.

Normark: Thank you professor, Botner. And we hope that we will get a telephone call now and that it has reached Professor Schmidt. Can you hear us on the phone?

Schmidt: Ah, yes, I can hear you just fine, thank you.

Normark: I guess it's around 8 o'clock, at your, in Canberra right now, is that true; or 9 o'clock?

Schmidt: Yeah, it's about 9 o'clock at night.

Normark: Yeah, so I'm sitting here in the beautiful session hall in Stockholm at the Royal Swedish Academy of Sciences and I guess, in front of me, I have around 100 persons representing Swedish and international media. I congratulate you once again, but I'm sure that there are a number of them that are eager to pose some questions to you. Are you ready for that Professor Schmidt?

Schmidt: I am and thanks to the Worldwide Web, I can actually see you guys as well. (laughter)

Normark: Okay. So, please.

Victoria: Hello, hi, Professor, my name is Victoria Dyring I am from Swedish television. First of all, we're live on air, right now also by this television thing. First of all, congratulations.

Schmidt: Thank you very much.

Victoria: How does it feel today?

Schmidt: Well, I was saying, it sort of feels like when my children were born, I feel kind of weak in the knees, very excited and somewhat, I guess, amazed by the situation. It's been a pretty exciting last half hour.

Victoria: You were one of the favorites on the betting lists. Did you expect this call?

Schmidt: No, I certainly did not expect it. I guess it's one of these things that you, you know, I guess occasionally people mention it, but you know, I guess it's one of these things, you just don't think it's probably ever going to happen.

Victoria: Okay. What are you planning for today?

Schmidt: Well, it's 9 o'clock at night tonight, so I think I will wander around and at some point try to go to sleep; we'll see how successful we are at that. (laughter) Tomorrow, I think, we're going to have to celebrate somehow, and I'm going to have to think hard and long about how to do that, but I'm looking forward to teach a cosmology class on this very subject tomorrow, so that should be interesting, and I'm going to think in the next couple of hours of what to do tomorrow.

Victoria: Good, will we be seeing you in Stockholm in December?

Schmidt: Most certainly. I grew up in Alaska, so I'm looking forward to the winter. (laughter)

Victoria: Good. Thank you very much.

Normark: Karen Boyce.

Karen: Hi, I'm Karen Boyce from the newspaper Dagens Nyheter and a big congratulations for the prize.

Schmidt: Thank you so much.

Karen: My question is about your feelings too, not the feelings today, but the feelings, I think, 13 years ago, when you realized that the supernova is actually accelerating away from us. What did you feel when you get that?

Schmidt: Well, Adam Riess and I were working very closely at the time and talking on the phone all the time, and trying to figure out this crazy result that was in our data. At that time, we knew we're in a race to measure what the supernovae were doing with Saul Perlmutter's team. But at that time we thought Saul Perlmutter's team was getting an answer that seemed sensible, which was that the universe was slowing down. Now, the fact that we were getting an answer that was exactly the opposite—it was speeding up and that the universe should not be speeding up—was pretty perplexing. So, we were, sort of, practically trying to sort out why, where we had gone wrong, and we could not make it go wrong. And so it was with a fair bit of trepidation that we ended up, you know, telling our group and then eventually telling the world that we had this crazy result: The universe seems to be speeding up. And we were hoping everyone would be nice to us, but we weren't, you know, it's one of these things that seem, kind of, too crazy to be right, so, I think we were a little scared.

Karen: And today, what do you think is the reason, what is the mechanism for this?

Schmidt: Well, I always looked to Einstein because Einstein didn't, he got a lot right, and so Einstein's idea that there is literally this, essentially an energy—that space itself has energy—it's the simplest reason that the universe could be speeding up. Because under Einstein's equations of general relativity—which is his lesser known theory, but his probably his biggest theory, 1915 to 1916—if space has energy, it causes the universe to accelerate. This energy pushes on itself and literally causes a runaway. And so that is the one model of dark energy that really makes solid predictions, and myself and many, many other people around the world have gone out and spent the last 13 years testing that prediction of Einstein. And thus far, every test we have made has come out perfectly in line with Einstein's original cosmological constant from 1917.

Normark: Do I have some more questions to Professor Schmidt. If not, thank you Professor Schmidt, and we're looking forward to see you here in Stockholm in December. Congratulations.

Schmidt: I look forward to it too.

Normark: So all that—any further questions that you would like to pose to the podium?

Karen: To the committee. This is a prize for observations, but the mechanism isn't actually proved yet, and with this logic, why didn't you reward Vera Rubin who filed observations reporting dark matter, which is not either proved, but is a much older observation that is equally revolutionizing.

Normark: You want to respond to that?

Botner: I can start and maybe Professor Brink can fill in. Now the reason why we decided to go for this prize this year is not only that has been made observations several years ago but also that these observations have since been confirmed by other observations. So, we are now quite certain that the observation of the accelerating expansion is correct. With regard to Vera Rubin, yes, she was one of the people, who is said to have discovered dark matter; she is not the only one, and there is a history behind it, which still has to be investigated. So, it is not as clear-cut as today's prize.

Brink: Right, I can continue with that. What is very special with this observation, is that it's really testing the basic theory of general relativity. So what's happening here is that we have a framework, a theoretical framework set up by Einstein, where we can really write down correct equations that we are checking. So this is not, there is no approximations, there's not, these are solid tests of Einstein's theory, and it comes out precisely right. And the fact that everyone was surprised about this acceleration, including us at the time, is that physicists are conservative—Einstein, and also Lemaître from France who was a physicist and a priest in the '20s and '30s later on—they had the cosmological constant, that's a logical possibility in the equations, and this is what we now have found. These observations have proved that there are other terms in Einstein's equations, which Einstein once proposed. We can even measure it very accurately.

Karen: So do you have any second line of observations supporting this cosmological constant?

Brink: Exactly, yeah.

Karen: Do you have any second line of…

Brink: Sorry, Do you have another… yes, what has happened after the 1990s, is that there are other kinds of observations by which you can also measure the same parameters. So, now there are satellite experiments and balloon experiments, which provide us with different tests of Einstein's theory, and they all, sort of, you know, fit in one point. So in that sense, it's a very solid observation; it's a very solid result even theoretically.

Botner: And if I may add one more thing, it's that we have a more recent observation that the universe is actually decelerating; it is, the expansion is slower at an earlier date. So if you look even further back than the supernovae they observed, towards the beginning of time, one discovers that the universe, when it was dominated by matter, then it was actually slowing down. But as the universe, as the matter dispersed, another form of energy dominates the universe, and that's the dark energy. And from that time on the expansion is actually accelerating. So, all the observation, even the observation which are more recent than the ones which are winning the prize today, are consistent in our understanding of the cosmology of the universe.

Normark: Another question there?

Boyd: It's Thomas Boyd with the Germany News Agency. Have you reached the other gentlemen, and were they well?

Normark: (laughter) We have reached Professor Riess, and we have not reached Professor Perlmutter, but I have only his cell phone. We hope that he is sound asleep and turned off his cell phone. All three laureates, I should say, are young. Saul Perlmutter is 52, and the other two are 44 and 42, respectively. So like last year, it's a pretty young group of laureates. If you have some additional questions? If not, I'd like to close this press conference.